2 3 2 D O C U M E N T 1 0 1 A U G U S T 1 9 2 0
carried out on an invariant (multiplied by [ / ]) does not furnish a quantity
of the character of a tensor:
(better written as ) is a mixed tensor;
(better written as ) also is a (covariant) tensor.
The product does not have the character of a
tensor,[2]
however. Might
you be thinking of the form
?
Even in this case, though, I cannot make any clear sense of your equations. In my
opinion, it would be very advisable if you kept the covariant forms clearly distin-
guished from the contravariant ones.
I am sending you the requested
paper[3]
under separate cover. It merely consti-
tutes an attempt at showing, specifically, that electric charges are possible with the
pseudospherical cosmological solution, without having to introduce new formal
constructions in all the equations (cosmological constant, special term for the
Hamiltonian function, which yields the cohesive forces required for constancy of
the electron’s spatial charges).
Whether the equations are derived from a law of variational principle or not
seems unessential to
me.[4]
But I emphasize that the paper’s content is just an at-
tempt, which I do not, by any means, back with full scientific conviction.
In great respect,
A. Einstein.
101. From Erwin Freundlich
Heidelberg, 12 August 1920
Dear Mr. Einstein,
I have completed the first series of experiments with the oven and am leaving on
vacation
tomorrow.[1]
The oven worked well and we have obtained good exposures
of the cyanogen band at 3883 Å in absorption at a dispersion of 1 Å = 1.2 mm, thus
sufficient for accurate
measurement.[2]
We are now having the oven modified a bit
in order to extend the lifetime of the graphite tube. I am in suspense about how the
line structure behaves and whether the results by Grebe and Bachem will be con-
firmed. In any event, I believe it will not be without interest sometime also to com-
pare directly the solar absorption spectrum with a terrestrial absorption
spectrum,[3]
whose source type, as relates to pressure, temperature, Doppler effect,
etc., is as clearly reproducible as with the oven. Important new tasks present them-
1 +
–g –q
g g